Ronald J. Rider, MBA
Copyright © 2016 Long International, Inc.
L ONG INTERNATIONAL
Long International, Inc. • 5265 Skytrail Drive • Littleton, Colorado 80123-1566 USA • Telephone: (303) 972-2443 • Fax: (303) 200-7180 • www.long-intl.com
Considerations for Identifying and
Analyzing the Critical Path
Ronald J. Rider, MBA
Table of Contents
1. INTRODUCTION ............................................................................................................................... 1 2. THE CRITICAL PATH DEFINED .................................................................................................. 2 3. IS THE CRITICAL PATH VALID?................................................................................................. 5 3.1 SCHEDULE QUALITY CHECKS TO CONSIDER .............................................................................7 4. STEPS TO CONSIDER FOR IDENTIFYING THE CRITICAL PATH ...................................... 9 5. CONCLUSIONS ............................................................................................................................... 16 List of Tables
Table 3-1
Schedule Quality and Deficiency Check Examples .................................................................. 7 List of Figures
Figure 2-1 Single and Multiple Critical Path Examples ............................................................................. 3 Figure 2-2 Critical Path Influenced by Constraint Example ....................................................................... 4 Figure 3-1 Sample Critical and Near-Critical Path Bar Chart Sort by Early Start Dates ............................ 6 Figure 4-1 P6 Menu Items to Create Critical and Near-Critical Path Codes ............................................ 10 Figure 4-2 Defined Critical and Near-Critical Path Code Menu............................................................... 11 Figure 4-3 Identify and Code Latest Finishing Activity Example ............................................................ 12 Figure 4-4 Identification of Driving Predecessors from Activity A9999 ................................................. 13 Figure 4-5 P6 Critical Path Grouping and Summarization Example ........................................................ 14 Figure 4-6 Final Critical Path Summary Example .................................................................................... 14 Copyright © 2016 Long International, Inc.
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Considerations for Identifying and
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1.
INTRODUCTION
The primary purpose of the time management function for a project is to plan, schedule, control,
and monitor the timely completion of the project’s scope of work. The standard method used
throughout the construction industry is to apply the Critical Path Method (CPM) scheduling
technique. The CPM scheduling technique is utilized by project management to plan, sequence,
forecast, and resource level the start and finish of work packages or schedule activities.
The all-important analysis tool provided by the CPM scheduling technique is the ability to
determine the critical activities that comprise the critical path. The critical path represents the
longest path or overall sequence of tasks and milestones that determine the project’s overall
duration. Activities not on the critical path will have some degree of float or slack time; activities
with available float or slack time can be delayed by the number float or slack days without
delaying the project’s overall finish date.
Project stakeholders must know, with a high degree of certainty, what the forecasted critical path is
in order to make sound and informed decisions relative planning the remaining work, applying
resources, dealing with potential delays, or accelerating portions of the project scope. CPM
scheduling programs allow users to select the activities that are considered to be on the critical
and/or near-critical paths by filtering the schedule data based on the longest path or low float value
criteria. The critical path and near-critical path output is typically in bar chart format or in tabular
report format.
Unfortunately, for large and complex engineering, procurement, and construction (EPC) projects,
the ability to quickly and accurately identify, summarize, and evaluate the critical path and nearcritical paths can prove difficult. It is not unusual for large EPC projects to contain multiple
process areas represented by tens of thousands of schedule activities, which makes identifying the
critical path and near-critical paths a burdensome endeavor. Further, the user is at the mercy of the
CPM scheduling program in terms of how the output of the schedule data is presented. In addition,
any deficiencies in the CPM model such as open-end activities and overuse of constraints further
complicate and muddle the process.
Therefore, it is up to the schedule analyst to use the confines within the CPM scheduling program
to better organize the schedule data in a format that the project participants can understand. The
purpose of this article is to discuss some of the challenges faced and steps to consider when
identifying and analyzing the critical and near-critical paths.
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Considerations for Identifying and
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2.
THE CRITICAL PATH DEFINED
The critical path is generically defined as the longest continuous path of activities, from the
progress date or data date to the final completion activity, which determines the overall duration
for a project. The activities that typically comprise the critical path are those activities with the
least amount of total float (i.e., zero or negative total float values). An activity on the critical path
is planned to not be started or completed until its logical predecessor activity or activities have
started or completed. If a critical path activity is delayed by one or many days, then any successor
activities including the project’s final completion activity will be delayed unless mitigations are
taken to recover the delay.
The following are two scenarios among many used to explain the rationale behind adopting a
certain critical path in a given situation:
a) Scenario No. 1 – A contractor may develop a critical path based on the
calculated result of the planned network logic and estimated activity durations
and other milestones included in the schedule; or
b) Scenario No. 2 – A contractor may develop a critical path based on the
collaborative input from the project manager, superintendent, subcontractors,
and/or others to force the critical path to go through certain activities that those
experienced people are satisfied with, because, based on prior history and
experience on similar projects, the criticality typically goes through this
work sequence.
The critical path can be represented by a single path or it could be represented by multiple parallel
paths. Figure 2-1 provides a simple example of a single critical path and multiple critical paths.
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Figure 2-1
Single and Multiple Critical Path Examples
Ordinarily, the critical path is represented by a continuous path of activities that is traceable from
the data date to the final completion activity as depicted in Figure 2-1. However, in certain
situations, the critical path may not be represented by a continuous path of activities. The start of
the critical path may begin at a point in time after the data date. For example, it is not uncommon
to find one or many critical path activities with an imposed start or finish constraint. Hence, the
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critical path is influenced by the constraint and the constraint may not allow the critical path to
fully trace back to the data date. Figure 2-2 is an example of a critical path that is influenced by an
activity start constraint.
Figure 2-2
Critical Path Influenced by Constraint Example
If the critical path is influenced by a constraint, as illustrated in Figure 2-2, the basis for the use of
the constraint that dictates that start of the critical path should be scrutinized for reasonableness.
Further, an understanding and rationale for using a constraint that influences the critical path
should be well documented and explained. This is typically found within the schedule basis
memorandum, which is a written document that describes, explains, and details how the CPM
schedule was developed and what assumptions were utilized.
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3.
IS THE CRITICAL PATH VALID?
Regardless of the size and complexity of a project schedule, it is important verify the validity of the
activities that constitute the critical and near-critical paths. The following two underlying
questions need to be answered:
1) What is the critical path?
2) Is the critical path reasonable?
The answers to these two questions may not be easy to determine. First, on large and complex
EPC projects spanning several years, the ability to quickly and accurately identify, summarize, and
evaluate the critical and near-critical paths can be cumbersome, especially when multiple activity
calendars are used resulting in non-uniform total float or slack values. It is not unusual for large
EPC projects to contain a mixture of process areas represented by tens of thousands of schedule
activities. The sheer magnitude of the schedule data makes identifying the critical and near-critical
paths difficult. Each area becomes an individual subproject which has its own individual driving
path and can include elements of engineering, procurement, fabrication, delivery, transportation,
installation, and commissioning and startup.
Second, the critical path is likely to change or shift multiple times. The CPM model is dynamic
which can allow the critical path to change from a group of activities shown in an initial baseline
schedule to different groups of activities during the course of a project’s execution depending how
much progress is achieved at a given point in time. It is not unusual for near-critical path activities
to be delayed beyond available float days or slack time, thus creating a new critical path of
activities driving project completion. Therefore, in order to complete a project timely, it is crucial
to correctly identify and manage closely the critical path along with any near-critical paths.
Third, the user is at the mercy of the CPM scheduling program in terms of how the output of the
schedule data is presented. Many of the standard output reporting features from the well-known
scheduling programs do not provide the user with a clear and easy-to-follow depiction of the
critical path or near-critical paths. The critical path work flow and logic links become cluttered
and can resemble a jumbled mass of spaghetti. Figure 3-1 is a screen shot of a sample bar chart
that includes critical and near-critical path activities sorted by early start dates.
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Figure 3-1
Sample Critical and Near-Critical Path Bar Chart Sort by Early Start Dates
As shown in Figure 3-1, the output can be messy, and the information to be conveyed to project
stakeholders may be unclear. The horizontal work flow from engineering, procurement,
fabrication, installation, commissioning, and startup is blurred, and the ability to understand the
relationships between the critical and near-critical paths is difficult to evaluate unless a tabular
predecessor and successor report is provided. The bottom line is that the capability to easily
evaluate and validate the various drivers to the paths is hampered. Ultimately, the message to
project participants is muddled.
Furthermore, the evaluation of the critical and near-critical paths can become more difficult if a
multitude of change orders and other impacts are introduced into the project schedule. In
addition, as discussed in the subsequent section, deficiencies contained in the schedule file could
invalidate the reasonableness of forecasted dates and float or slack values unless the deficiencies
are corrected.
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3.1
SCHEDULE QUALITY CHECKS TO CONSIDER
Schedule quality checks should be considered a mandatory requirement during the critical path and
near-critical path validation process. Any deficiencies or errors contained within the schedule, if
not corrected, could substantially affect the accuracy of the project’s forecasted critical and nearcritical paths. Therefore, checking the integrity of the CPM model for obvious deficiencies is
needed to ensure that that the model is accurate and reasonable. Table 3-1 is a listing of some
common quality and deficiency checks for consideration.1
Table 3-1
Schedule Quality and Deficiency Check Examples
Item
No.
Quality/Deficiency Check
Description
1
Missing Original Scope
Review contract documents and estimate files to verify to the extent
possible that all required contract scope is represented in the
schedule file. A few commonly missed activities include submittal
process, owner or contractor approvals, customs clearance, quality
assurance and control, concrete cure period, pre-commissioning
process, punchlist requirements, and turnover process. Correcting the
schedule file for missing original scope activities could change
forecasted dates and potentially alter the critical and near-critical paths.
2
Excessive Number of Open-End
Activities
Open-end activities are defined as activities that have no predecessor
activity, no successor activity, or both. Good scheduling practice
requires all activities to have at least one predecessor and one
successor except for: (1) the project start activity (no predecessor
activity); and (2) project completion activity (no successor activity).
A large number of open-end activities will create erroneous float as
well as cause forecasted activity sequences to be potentially
performed illogically (i.e., float from the data date to project finish).
Correcting open-end activities will change float values which may
affect the forecasted dates for critical and near-critical activities.
3
1
Large Number of High Float
Activities
A project schedule with a large number of high float activities can be
an indication of underlying schedule quality problems that may need
to be fixed. Schedule paths with high float values are typically the
result of open-end activities discussed in Item 2 above, artificially
constrained activities, or lack of work flow optimization on part of
the contractor. Schedules with large numbers of high float activities
should be thoroughly examined and corrected for any open-ends or
missing logic links that would better optimize the forecasted dates
and float values.
Long International has developed a standardized procedure and template for the specific elements needed for
schedule quality assurance. See Long International’s article, “Schedule Quality Assurance Procedures,” for
more detailed discussion. A copy of this article is located at Long International’s website (www.long-intl.com).
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Considerations for Identifying and
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Item
No.
4
Quality/Deficiency Check
Inconsistent Use of Schedule
Calculation Modes
Description
Retained logic is the default calculation mode for nearly all schedule
programs. Another type of scheduling calculation mode is progress
override, which allows out-of-sequence work activities to progress
without delay and not wait until logical predecessors are complete.
Progress override is based on the premise that a contractor has all the
necessary resources in terms of labor, equipment, and supervision
that would allow for working in multiple areas concurrently.
Many contractors will use progress override as a way to update a
schedule without having to spend the time correcting out-ofsequence work. In many cases, contractors will switch schedule
calculations mode during the project from retained logic to progress
override and then back to retained logic without reason or
justification. Inconsistent use of schedule calculation modes could
be deemed a deficiency, and after changing schedule calculation
modes to be consistent, the critical and near-critical paths
may change.
5
Overuse of Constraints
Another important schedule quality check is to determine the
number of constraints applied to activities. Constraints are defined
as restrictions imposed on either the start or finish of an activity that
may artificially lock down a schedule and prevent a schedule to
naturally “flow” during a forward and backward pass calculation to
determine activity float values and the critical path of the schedule.
While some constraints may be needed due to contractual
requirements, the overuse of constraints should be avoided. Overuse
of constraints could cause discontinuity for the critical and nearcritical paths between the data date and the final completion activity,
thereby masking the true critical path for the schedule. Fixing the
problems caused by overuse of constraints could alter the critical and
near-critical paths.
6
Misapplication of Calendars
When activities are erroneously assigned to the wrong calendar, such
as a 5-day work week rather than a 7-day work week, the critical
path calculations may be incorrect.
The schedule calendar
definitions should be reviewed and then the critical and near
activities should be evaluated to verify which calendar is applicable
to each activity. Evaluations should be made of the calendar usage
as to whether the calendar assignment is reasonable based on where
each task is performed and which project resources are assigned to
complete the work.
If a schedule model contains one or many of the issues identified in Table 3-1, then it is likely that
these issues will materially impact either directly or indirectly the reasonableness of the critical and
near-critical paths. If not corrected, important project decisions including the application of limited
resources may be applied incorrectly. Correcting the issues above could involve an iterative
process requiring time and effort by the project team.
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Considerations for Identifying and
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4.
STEPS TO CONSIDER FOR IDENTIFYING THE CRITICAL PATH
After the above schedule deficiencies or errors have been considered and corrected as suggested in
the prior section, several steps are recommended to aid in the identification and analysis of the
critical and near-critical paths. These steps provide structure, organization, and consistency to the
overall process as well as allow flexibility to the user for sequencing the information in a
presentation format that better communicates the information to project stakeholders. No matter
the size of the schedule file, the steps detailed below are used to identify and analyze the critical
path, although some steps may require modification and additional steps may be added depending
the schedule data contained within a particular schedule file. The steps below are based on using
the scheduling software Primavera P6 Professional (“P6”). The following steps are provided
for consideration:
1) Step 1: Select and Copy the Appropriate Schedule File – The initial step is
to ensure that the schedule file being analyzed is the correct and most update-todate version. For analysis purposes, it is highly recommended that a copy is
made the original schedule file in order to preserve the original. In addition, a
tracking table should be prepared to log original versus copy schedule file
names along with summary schedule data for each file (i.e., P6 file name,
number of activities, forecasted overall finish date, schedule calculation mode,
budget resource totals, etc.).
2) Step 2: Create Critical and Near-Critical Codes – The P6 program provides
almost limitless ability to create, add, and assign codes to schedule activities.
These codes can be assigned and applied globally (i.e., across all projects
contained within the P6 database), defined Enterprise Project Structure (“EPS”)
groups, or individual projects only. Figure 4-1 shows the P6 menus used to
create critical and near-critical path codes.
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Considerations for Identifying and
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Figure 4-1
P6 Menu Items to Create Critical and Near-Critical Path Codes
In the example shown in Figure 4-1 above, the critical and near-critical path
codes were created for global use.
3) Step 3: Define Critical and Near-Critical Path Codes – Once the global
category has been created, the next step is to define the critical and near-critical
path codes for the global category. The defined codes are used as groups that
provide structure and flexibility when identifying and coding activities on these
paths. Figure 4-2 shows sample critical and near-critical path codes to be
applied to schedule activities.
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Figure 4-2
Defined Critical and Near-Critical Path Code Menu
As shown in Figure 4-2, the critical paths were defined using reference codes
A0 through AD. The codes B0 through BC were used to define near-critical
path activities. Also, within the P6 program, the critical and near-critical path
codes are organized for sequencing the codes as desired by the user.
4) Step 4: Identify and Code the Latest Finish Activity – The next step is to
identify and code the latest finishing work activity within the schedule file. To
illustrate, a simple process plant project schedule file was prepared and the
schedule data was sorted by early start. Figure 4-3 is an activity listing of the
sample project.
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Figure 4-3
Identify and Code Latest Finishing Activity Example
In Figure 4-3, the latest finishing activity was identified as activity ID A9999
(Start Producing Product), and the critical path code “AD” as assigned to
this activity.
5) Step 5: Identify and Code Driving Predecessor Activities – From the latest
finishing activity A9999, the “trace-logic” feature in the P6 program was
utilized to identify the driving predecessor activity that controls the forecasted
dates for activity A9999. Once the driving predecessor has been identified, a
driving critical path code is assigned to the predecessor activity. Figure 4-4 is
an illustration of this process.
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Figure 4-4
Identification of Driving Predecessors from Activity A9999
In Figure 4-4, the “trace-logic” feature in the P6 program is designated by the
“Driving” box with a check mark shown in the Predecessors tab below the bar
chart information (see yellow highlighted box). Using this process, the
driving predecessors for the path can be traced and coded all the way back to
the data date.
6) Step 6: Organize and Summary the Critical Path – After assigning critical
path codes to all of the driving predecessors back to the data date, the schedule
data can then be organized and summarized by descending critical path codes
for further examination for reasonableness. Figure 4-5 is an example of the P6
grouping and sorting menu options for this step.
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Figure 4-5
P6 Critical Path Grouping and Summarization Example
Based on the criteria in Figure 4-5, a summary of the critical path for the project
can be generated. Figure 4-6 is the final generated critical path chart for the
sample project.
Figure 4-6
Final Critical Path Summary Example
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The same steps can be applied when determining near-critical paths. Based on the output from
Figure 4-6, the user can then further examine the path for reasonableness. Areas to check and
consider include but are not limited to the following:




Reasonableness of constraints on the path, if any;
Unusually long gaps or non-work periods between critical path activities;
Questionable logic links on the path (i.e., use of start-to-finish links and
negative lags); and
Consistency with contemporaneous project records.
It is recommended that key project participants perform joint reviews of the final critical path
summary information to ensure that all parties are in agreement with the results. Based on these
reviews, the critical path can be validated. Further, it is recommended that the results be cross
checked with other documents such as the project charter, Front-End Engineering Design (FEED)
documents, estimate information, and schedule basis memorandum information.
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5.
CONCLUSIONS
The identification and analysis of the critical path can be challenging especially when evaluating
large and complex EPC projects with tens of thousands of activities. The process is complicated
by some of the standard graphical and tabular presentation features contained within the schedule
software. However, simple steps can be applied for coding the critical and near critical activities to
better identify, organize, and summarize the critical and near critical activities for further analysis
and assessment of their reasonableness.
About the Author
Ronald J. Rider, M.B.A., is a Principal with Long International and has over
25 years of experience in construction project management and contract dispute
resolution. Mr. Rider has performed CPM schedule development, project cost
control, cost and labor hour variance modeling, impact identification and
causation, change order pricing and resolution, retrospective schedule delay,
time extension and acceleration analyses, concurrent delay assessments and
damages quantification. Mr. Rider’s work experience includes petrochemical,
oil refinery, power, commercial, industrial, road/highway, transit,
hospital/medical, airport and casino projects. He holds a B.S. degree in
Construction Management from Colorado State University and an M.B.A.
degree from the University of Colorado. Mr. Rider is based in Littleton,
Colorado and can be contacted at rrider@long-intl.com and (303) 346-5836.
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